The present invention relates to a differential assembly and more particularly to a differential assembly having a biasing preload to synchronize shaft speeds.
Differential assemblies are well known in the art. In differential assemblies driving rear mounted axles, a limited slip device may be incorporated to limit the amount of differential speed rotation of the output shafts. Commonly a multiple-disc clutch pack is disposed between one or each of the side gears and the differential case. The differential case may have axial grooves or splines to prevent rotation and allow axial displacement of friction discs of the clutch pack. Similarly, clutch plates may be splined onto the side gears and interposed between the friction discs. A spring is normally disposed between the side gears to urge the side gears apart and cause frictional engagement in the clutch pack. The friction between the friction discs and clutch plates limits the amount that the side gears may slip relative to the differential case. A gerrotor pump or other hydraulic pumping mechanism may also be employed to selectively control the clutch pack. However, a limited slip device is undesirable and unnecessary in differential assemblies arranged in axle assemblies driving steerable wheels. Rather, differential assemblies in front steerable wheel end assemblies employ a traditional differential assembly. Note FIGS. 2-4 which show a conventional standard differential assembly with a flat thrust washer disposed between the side gears and differential case. These flat thrust washers are machined to within fine tolerances to ensure that no preload is induced to the differential case.
In four wheel drive axles assemblies it is desirable to selectively engage the front steerable axle shafts. Normally the rear axle assembly is constantly driven and the front axle assembly is selectively driven during four-wheel drive conditions. Typically, a transfer case is employed to transfer driving torque from the drive shaft to the front axle assembly. Once the transfer case is engaged and the front differential case driven, the wheel hub locks are engaged to complete the four-wheel drive path.
Ideally, in perfect conditions when the transfer case is engaged and brings the front axle differential case up to speed, both half shafts will be spinning at road speed coincident with the wheel end hub. The wheel end hublocks may then be engaged to commence four-wheel drive. However, in differential assemblies, there is inherently an unequal drag force on one of the side gears relative to the other side gear. Therefore, when the transfer case engages the front axle assembly to drive the differential case, the half shafts will rotate at different speeds. The extreme case is where one half shaft remains stationery while the other half shaft rotates at twice road speed. Consequently, it is difficult to engage the wheel end hublocks and may result in ratching of gears and splined teeth when the hublocks are engaged. Therefore, it is desirable to ensure each differential side gear and half shaft are running at the same speed when the wheel end hublocks are engaged without employing an undesirable conventional limited slip device.
The object of the present invention is to add some pre-load or bias to a standard differential case. If the bias in the differential case is greater than the difference in drag forces acting on the side gears it will prevent the differential gears from spinning thereby ensuring that both half shafts are rotating at the same speed. Having both half shafts rotating at the same speed will minimize any ratching of any gears or splined teeth when the hublocks are engaged. A beveled thrust washer is disposed directly between each side gear and the differential case. The beveled thrust washer induces a preload between the side gears and the differential case. The preload is sufficient to overcome any differences in drag forces acting on the output shafts. Consequently, when the transfer case in engaged to bring the differential case up to speed, the output shafts will rotate at the same speed coincident with vehicle running speed. Thus, the wheel end hublocks may be engaged.